Focus: Freezing into Unconsciousness

Just before you fall asleep for surgery, your brain may undergo a sudden phase transition, similar to water freezing.

Before a surgeon can unsheathe her scalpel and plunge into her work, she has to be absolutely sure the patient is asleep. In the December Physical Review E, researchers describe a new theoretical model of the brain that suggests that the general anesthetics used by surgeons in the operating room may trigger a sudden phase transition between the waking and unconscious states. The research may point the way to new tools that remove all uncertainty about a patient’s conscious state during surgery and could even be used as “sleep detectors” for air traffic controllers and truck drivers.

Determining the exact moment that sleep begins is a tricky business. One standard method is to measure electroencephalograph (EEG) signals from the brain using electrodes on the scalp. As the anesthesia takes effect, the small electrical current that passes between neurons increases to a peak and then falls as the patient loses consciousness. A similar rise and fall of the EEG power occurs when the patient wakes up. Although detecting these peaks in the EEG is a reliable indicator of sleep, it isn’t perfect. “Occasionally you hear these horror stories about patients who are aware during surgery,” says physicist Moira Steyn-Ross of the University of Waikato in New Zealand.

Instead of a gradual descent into unconsciousness, Steyn-Ross and her collaborators suspected that anesthesia actually induces a sudden phase transition between wakefulness and sleep, in the same way that chilled water freezes. Physicists can tell exactly when the water-to-ice phase transition occurs because the typical separation of two molecules suddenly switches from zero–there is no characteristic separation of the randomly distributed liquid water molecules–to the distance between two molecules in ice’s crystal lattice. The characteristic separation of molecules is the “order parameter” of freezing water.

If falling asleep is also a phase transition, there should be a relevant order parameter. Then, by measuring this parameter, an anesthesiologist could determine exactly when a patient is about to lose, or regain, consciousness. But first, Steyn-Ross and her colleagues had to prove that falling asleep is truly a phase transition.

To test the phase transition hypothesis, Steyn-Ross used a detailed mathematical model created by Swinburne University of Technology’s David Liley to simulate the electric currents exchanged by neurons in a millimeter-thick cylindrical column of the cortex called a macrocolumn. “The model predicts that as the concentration of general anesthetic is increased, there will be a sudden phase transition into unconsciousness which will be signaled by a sharp increase in EEG power,” explains co-author Alistair Steyn-Ross. The predicted power surge is seen in the EEGs of anesthetized patients, says Steyn-Ross, who takes this as strong evidence for the phase transition theory.

“They have built a fairly consistent story that anesthesia induces a phase transition,” says psychologist Scott Kelso, director of the Center for Complex Systems at Florida Atlantic University in Boca Raton, but he doubts that the raw EEG power is the order parameter of consciousness. Kelso believes that the true order parameter will only be discovered by making detailed comparisons between the theory and the complicated time- and location-dependent patterns of brain activity triggered by anesthetics. The Steyn-Rosses plan to address these questions in an upcoming paper.